Semiconductor components based on having silicon carbide as a base material are continuously developed to be used in connection with high temperatures, high power applications and under high radiation conditions. Under such circumstances, conventional semiconductors do not work satisfactorily. Evaluations indicate that SiC semiconductors of power MOSFET-type and diode rectifiers using SiC are able to operate over a greater voltage and temperature interval, e.g. up to 650-800.degree. C., and show better breaker properties such as lower losses and higher working frequencies but nevertheless have a volume 20 times smaller than corresponding silicon components. These possible improvements are based on the favorable material properties that silicon carbide possesses in relation to silicon, such as a higher breakdown field (up to 10 times higher than silicon), a higher thermal conductivity (more than 3 times higher than silicon) and a higher energy band gap (2.9 eV for 6H-SiC, one of the crystal structures of SiC).
As SiC semiconductor technology is relatively new and in many aspects non-optimized, there are many critical manufacturing problems that are to be solved before SiC semiconductor devices may be realized experimentally and manufactured in large quantities. This is especially true for components intended for use in high-power and high-voltage applications.
One of the difficulties to overcome when manufacturing high voltage diodes or other types of semiconductor components comprising a voltage absorbing pn junction is to produce a proper junction termination at the edge of the junction. The electric field across the pn junction is very high, when a reverse voltage is applied across the pn junction.
A high reverse bias generating a strong electric field at the edge of the pn junction implies a great risk of voltage breakdown or flash-over at the edge of the junction. In the region of the component surface, where the pn junction reaches the surface, an increase of the electric field arises compared with the conditions existing within the bulk of the junction. This is due to the changeover from more homogeneous conditions inside the crystal of the component to the abrupt step out of the crystal lattice at the surface. This effect makes it very important to reduce the field concentration where the junction reaches the surface. Combined with efforts to passivate the surface of the component, measures are taken to flatten the electric field at the surface e.g. by acting on how the pn junction emerges at the surface. As an example, it is known from silicon power components to lap (grind or saw) the surface of the edge to a certain angle in relation to the pn junction to thereby flatten the field. Another known technique is to gradually decrease the doping of the conducting area around the junction, so that the doping is reduced towards the outermost edge of the junction (so called Junction Termination Extension, JTE) in order to elimininate field concentration at the edge of the junction. These methods, known from silicon technique, are difficult to apply to components based on silicon carbide due to its being a very hard material; similarly doping through diffusion is extremely difficult, and so on.
The above-mentioned problems have not been solved for pn junctions in SiC. Many of the problems to be solved when developing semiconductor components from SiC are reminiscent of those prevalent at the beginning of the development of the silicon components. Yet, the techniques applicable to silicon cannot be utilized when solving the specific problems related to production of SiC semiconductor components. As an example, doping through diffusion is not feasible for SiC, as diffusion coefficients are negligable below 2270.degree. K. Also, ion implantation of doping elements, a common technique when manufacturing Si components, is difficult to master and not fully developed for SiC.
High voltage diodes from 6H-SiC with epitaxially formed pn junctions and Schottky junctions have been done experimentally (see e.g. M. Bhatnagar and B. J. Baliga, IEEE Trans. Electron Devices, vol. 40, no. 3 pp 645-655, March 1993 or P. G. Neudeck, D. J. Larkin, J. A. Powell, L. G. Matus and C. S. Salupo, Appl. Phys. Lett. vol 64, No 11, 14 March 1994, pp 1386-1388). Some of the problems related to SiC devices have thus been solved, but nothing is discussed about the problems connected to electric field concentration at the edges of the junction.
The electric field may be reduced at the edge of the pn junction by-applying a semi-isolating layer to the edge of the junction of a SiC component. Such a solution is described in document PCT/SE94/00482.
Any method or device used to accomplish a semiconductor component corresponding to the principle of Junction Termination Extension at a pn junction composed of Si is not known for use with a component, in which SiC constitutes the base material of the junction. This invention aims at providing a voltage absorbing edge at a pn junction with a structure similar to JTE of a Si component.
The term SiC is used in the following text to refer to any of the principal crystal polytypes of this material known as 6H, 4H, 2H, 3C and 15R.